GeezerVison

What

Adds a Troubleshooting section to the README covering the two most frequently repeated support questions for displays without hardware (DDC) control:

1. Screenshots / screen recordings / screen sharing look dim — explains that shade-overlay dimming is captured while gamma-table dimming is not, and how to fix it by turning off Avoid gamma table manipulation.
2. Brightness jumps suddenly at a point on the slider — explains the software/hardware dimming handover on non-DDC displays and the same fix.

Why

The README has no Troubleshooting/FAQ section, yet these two questions recur often in Issues and Discussions and are answered the same way each time by the maintainer, e.g. #1446, #1252, #1388, and discussions #1453 / #1189. Documenting the cause and the one-setting fix in the README should reduce repeat questions and help users self-serve.

This is docs only (README.md); no code changes. It is complementary to the open code fix in #1877 (which excludes the overlay from capture for real displays) — the docs also cover the gamma default and the slider-jump case, which that PR does not.

Testing

Documentation change only. Rendered Markdown reviewed for correct formatting and links.